Big Sky Science Partnership Glacial Processes Field Trip Assignment
June 22, 2010
This field trip focuses on examining the sedimentary record of different depositional
environments associated with the last glaciation and subsequent deglaciation in the Arlee
Valley of western Montana. The field trip will consist of six stops. The six stops will
focus specifically on different styles of sediment deposition: 1) deposition of sediment by
glacial ice; 2) deposition of sediment by a glacial outwash stream; and 3) deposition of
sediment at the bottom of a deep lake.
I)
Deposition of sediment by glacial ice. Stops 1 and 2 will focus on examining
the record of sedimentation by ice. Glacial ice is capable of flowing like a
very viscous(stiff) fluid. The high viscosity provides the ice with significant
strength that allows it to erode bedrock efficiently and transport very large
boulders. The high viscosity also prevents the ice from sorting its sediment
load much before it is deposited.
Focus on the following observations to test this hypothesis:
a) Stop 1: Approximately how far (order of magnitude) did the glacial ice
travel from its source to its terminus? How might this value compare to
distances traveled by lower viscosity natural fluids such as streams, and
what does it suggest about the viscosity of the ice
b) Stop 1: Sketch and describe the topography shaped by glacial erosion in
Finley Creek and the southern end of the Mission Range. Use the reverse
side of this sheet if needed. How would you expect this topography to
compare to similar regions that are eroded only by streams?
c) Stop 1: Sketch and describe the shape of the sediment deposit left by the
glacier at its maximum advance (i.e., the terminal moraine)? What does
the shape of this deposit suggest about the ability of the glacier to organize
the sediment that it carries?
d) Work as a team to gather some basic sedimentologic data from the clasts
in the moraine:
i. Using a tape measure, estimate the intermediate diameter of 20
clasts on the moraine surface and describe the shape of the clast
using one of the following categories: angular, subangular,
subround, round. Try to select the clasts randomly; do not simply
estimate the diameter of the biggest clasts
1)
2)
3)
4)
5)
11)
12)
13)
14)
15)
6)
7)
8)
9)
10)
16)
17)
18)
19)
20)
II)
Deposition of sediment by glacial outwash streams. Stops 3 and 4 will focus
on examining the record of sedimentation by flowing water. Relative to
glacial ice, water flows as a much lower viscosity fluid capable of depositing
a well-sorted sediment load.
Focus on the following observations to test this hypothesis:
a) Stop 3: Describe in your own words the overall shape and surface topography
of the McClure outwash fan relative to the shape of the Finley Creek moraine.
b) Stop 4: Carefully sketch the arrangement of clasts in one of the exposures in
the road cut. Is there anything about the arrangement of clasts that suggests
deposition by a moving flow of water?
a) Like you did at Stop 2, work as a team to gather some basic
sedimentologic data from the clasts in the outwash stream deposit. Using a
tape measure or ruler, estimate the intermediate diameter of 20 clasts and
describe the shape of the clast using one of the following categories:
angular, subangular, subround, round.
1)
2)
3)
4)
5)
11)
12)
13)
14)
15)
6)
7)
8)
9)
10)
16)
17)
18)
19)
20)
How do the sorting and shape of the clasts in the glacial moraine compare to those in the
glacial outwash deposit?
How do you explain the observed differences in clast sorting and shape?
III)
Deposition of sediment in a deep lake. Stops 5 and 6 will focus on examining
the record of sedimentation in a deep glacial lake. Fine-grained sediment is
deposited at the bottom of a deep lake in two basic ways: 1) slow suspension
settle-out through the water; and 2) as bottom-hugging turbulent mixtures of
sediment and water, called ‘density currents’
Focus on the following observations to test this hypothesis:
a)
Stop 5: Describe the occurrence of the glacial erratic. Does it occur
as part of more extensive sedimentary deposit or as an isolated
clast? Given that this site was formerly situated on the floor of
glacial Lake Missoula, how do you explain the occurrence of this
clast?
b)
Stop 6: Carefully examine the structures present in the fine-grained
part of the outcrop. Find a part of the outcrop that consists of welldefined layers an inch or less thick. How fine is the grain-size in
these layers? Notice that each of the layers can be traced as far
laterally as the outcrop permits. What do the fine grain size and
lateral continuity of these deposits suggest about how they were
deposited?
c)
Stop 6: Now find part of the outcrop that consists of very fine sand
containing ripples. Carefully sketch the ripples in the space below:
d)
Stop 6: Now look for sedimentary structures that consist of a layer
of seemingly chaotic fine-sediment. Carefully sketch one or two of
these layers. How might a structure like this form? What does it
suggest about the nature of the lake bottom as sediment was being
delivered?